Long-term growth of murine mast cells from bone marrow-derived precursors is dependent upon the continuous presence of a mast cell growth factor (MCGF) contained in media conditioned by mitogen-activated splenic leukocytes or the murine lymphoma LBRM-33 cells, or in culture supernatant of the myelomonocytic leukemia WEHI-33 cells. Cultured mast cells show characteristic mast cell granules, exhibit surface IgE receptors and contain a number of mast cell products including histamine. Biochemical analysis revealed MCGF as a glycoprotein with molecular weight of 29,000 that showed substantial charge heterogeneity upon isoelectric focusing. MCGF is distinct from interleukin-2 (IL-2) as well as macrophage and granulocyte colony stimulating factors (M-CSF, GM-CSF and G-CSF). Analysis of murine bone marrow cells showed mast cell precursors (MCP) responding to MCGF to be low-density cells (1.018 to 1.033 g/ml) with a rapid sedimentation rate of 5 to 8 mm/h. MCP are thus distinct from T cells and T-cell precursors exhibiting a density of 1.068 to 1.079 g/ml and a sedimentation rate of 2.5 to 4.5 mm/h. Long-term cultured mast cells are immature cells. Maturation can be achieved by cultivation in the absence of MCGF on top of adherent layers of Dexter-type long-term marrow cultures overlaid with a layer of 0.6% soft agar. Maturation is demonstrable by changes in morphology, ultrastructure, increased content of histamine and various other enzymes, as well as density of surface IgE receptors. Further characterization of MCP and the maturation inducing elements in murine bone marrow is under way.